Equilibrium dialysis experiments have shown that the vasopressing analogue, methionyl-tyrosyl-(14C)phenylalaninamide, interacts with each of bovine neurophysins I and II specifically, having characteristics of binding similar to, but distinguished in important ways from, those of the intact peptide hormones. The analogue peptide serves as a valuable model and has allowed the preparation of an affinity matrix, to be used in future studies of neurophysin-hormone interacting complexes. Sedimentation-equilibrium studies have shown that ligands affect aggregation properties in such a way as to suggest a relationship between the state of aggregation of the neurophysins and the quantitative character of the interaction with hormones. Neurophysins were found to be highly susceptible to inactivation by disulfide interchange. Combined with the observation of reduction of this susceptibility by the ligand Met-Tyr-Phe amide, the results are consistent, among other possibilities, with the hypothesis that the neurophysins are biosynthesized as a part of a larger precursor protein and that part of the additional polypeptide of this percursor may correspond to sequences analogous to those of the neurohypophyseal peptide hormones.